Tunable single-phase magnetic behavior in chemically synthesized AFeO3-MFe2O4 (A = Bi or La, M = Co or Ni) nanocomposites

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PAPER

Cite this:Nanoscale, 2018, 10,

22990

Received 27th August 2018, Accepted 14th November 2018 DOI: 10.1039/c8nr06922k rsc.li/nanoscale

Tunable single-phase magnetic behavior in

chemically synthesized AFeO

₃

–MFe

₂

O

₄

(A = Bi or

La, M = Co or Ni) nanocomposites

†

T. Sarkar,

*

a

G. Muscas,

b

G. Barucca,

c

F. Locardi,

d

G. Varvaro,

e

D. Peddis

e

and R. Mathieu

a

The properties of magnetic nanocomposites rely strongly on the interplay between those of the constitu-ent componconstitu-ents. When the individual componconstitu-ents themselves are complex systems belonging to the family of correlated electron oxide systems which typically exhibit exotic physical properties, it becomes nontrivial to customize the properties of the nanocomposite. In this paper, we demonstrate an easy, but eﬀective method to synthesize and tune the magnetic properties of nanocomposites consisting of corre-lated electron oxide systems as the individual components. Our method is based on a novel synthesis technique by which the two components of the nanocomposite can be directly integrated with each other, yielding homogeneous samples on the nanoscale with magnetic behavior reminiscent of a single phase. We illustrate our method using multiferroic BiFeO3(BFO) and LaFeO3(LFO) as the major phase (i.e.,

matrix), and MFe2O4(M = Co 2+

or Ni2+) as the embedded magnetic phase. Furthermore, we show that by a proper selection of the second phase in the nanocomposite, it is possible to customize the magnetic pro-perties of the matrix. We illustrate this by choosing CoFe2O4and NiFe2O4, two oxides with widely diﬀering

magnetic anisotropies, as the embedded phase, and demonstrate that the coercivity of BFO and LFO can be increased or decreased depending on the choice of the embedded phase in the nanocomposite.

1. Introduction

Magnetic nanocomposites with tailored properties cover a wide variety of diﬀerent materials and material combinations1

that are of huge scientific and technological interest. The wide range of applications of magnetic materials include bio-medical applications and catalysis,2–9heavy metal removal for wastewater treatment,10–12 electromagnetic wave absorption and electromagnetic interference shielding,13–16 in energy devices,17and for obtaining improved mechanical18and physi-cal19properties, such as room temperature giant magnetoresis-tance.20Magnetic nanocomposites consisting of strongly

corre-lated electron oxides21as the individual components are par-ticularly interesting owing to their cross-correlated electronic and magnetic properties. They exhibit a multitude of exotic and functional phenomena, such as high-Tc

superconduc-tivity,22Mott states,23spin-liquid states,24colossal magnetore-sistance,25 and multiferroicity.26 The complexity further increases when the particle size is reduced to the nanoscale regime,27–32 and, furthermore, when two such materials are integrated with each other to form nanocomposites.33In such complex systems, the capability to control and tune the pro-perties is extremely important for extending their application in diﬀerent technological fields. Among those, BiFeO3 (BFO)

and LaFeO3(LFO) are promising perovskite materials

exhibit-ing ferroelasticity and high-temperature antiferromagnetism near 700 K,34,35 with BFO also being ferroelectric below 1100 K.35It is, thus, very relevant to tune and tailor the struc-tural and magnetic properties of these materials in the form of nanocomposites, and bring forth stronger magnetic and/or polar orders and enhanced magnetoelectricity.36

In recent years, ferrite-based composites have been increas-ingly attracting attention due to their diverse applications, and several novel synthesis methods have been reported. For example, core–shell α-Fe2O3/CoFe2O4 composites synthesized

using a solvothermal process exhibited enhanced microwave

†Electronic supplementary information (ESI) available. See DOI: 10.1039/ c8nr06922k

a_{Department of Engineering Sciences, Uppsala University, Box 534,}

SE-75121 Uppsala, Sweden. E-mail: tapati.sarkar@angstrom.uu.se

b_{Department of Physics and Astronomy, Uppsala University, Box 516,}

SE-75120 Uppsala, Sweden

c_{Department SIMAU, University Politecnica delle Marche, Via Brecce Bianche,}

Ancona, 60131, Italy

d_{Dipartimento di Chimica e Chimica Industriale, Università degli Studi di Genova,}

Via Dodecaneso 31, Genova, 16146, Italy

e_{Istituto di Struttura della Materia}_{– CNR, Area della Ricerca di Roma1,}

Monterotondo Scalo, RM, 00015, Italy

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absorption properties.37 Fe-Based composites derived from Fe3O4/Prussian blue composites were synthesized using a

con-trollable carbothermal route, and the magnetic and dielectric properties were studied.38 Lv et al. reported the synthesis of quaternary Fe0.5Ni0.5Co2O4 composites via a solvothermal

process followed by annealing.39FeCo nanoparticles embedded in nanoporous carbon composites were synthesized using a novel method involving thermal conversion of an Fe3O4/metal–

organic framework.40In this paper, we report an easy, low cost, and effective method to synthesize and tune the magnetic pro-perties of nanocomposites where the major phase (i.e., matrix) is composed of a multiferroic material. Conventionally, matrix-based nanocomposites are synthesized by dispersing magnetic nanoparticles in a bulk non-magnetic matrix. Here, we follow a different approach where the matrix itself is nanosized and magnetic (in our case, multiferroic BFO and LFO). The individ-ual matrices of BFO and LFO require different synthesis con-ditions. Nevertheless, our synthesis method is general, and applicable to several matrices. It is worth noting that BFO and LFO are complex systems, not only in terms of their correlated physical properties, but also as far as their synthesis is con-cerned. We specifically chose such complex systems owing, not only to their extremely interesting physical properties, but also to prove the robustness of our method, and show that the syn-thesis technique can be successfully used to prepare complex systems. Both BFO and LFO were synthesized using a modified sol–gel technique, albeit with differences in specific details that are elaborated in section 2.1.2.

To customize the magnetic properties of the nanocomposites, two diﬀerent spinel ferrites with diﬀerent anisotropies (i.e., CoFe2O4 (CFO) and NiFe2O4 (NFO)) have been chosen as the

embedded phase. It is also important to note that our samples are diﬀerent from epitaxial heterostructures36 or samples pre-pared by a templated growth of a magnetic phase in a matrix,41 where the two component phases are, by design, spatially segre-gated. The chemical synthesis route that we follow yields samples that are expected to yield a more homogeneous distri-bution, in the nanoscale, of the embedded phase (CFO or NFO) within the matrix (BFO or LFO) than what is generally achieved using a simple physical mixing of the two phases,33 the latter often tending to have large clusters of the two phases. This would, in turn, allow for better magnetic coupling between the two phases in our samples, as is indeed evidenced by our mag-netic measurements (elaborated in later sections).

In the present study, we present the detailed synthesis, structural, and microstructural characterization, and demon-strate the tunability of the single-phase-like magnetic behavior of the samples.

2. Synthesis and experimental

techniques

2.1 Synthesis

2.1.1 CoFe2O4 and NiFe2O4. Cobalt ferrite (CFO)

nano-particles were prepared by a sol–gel self-combustion process.

The details of the self-combustion process and thermo-gravimetric analysis have been published elsewhere.42,43 Briefly, 6.7 mmol of Fe(NO3)3·9H2O and 3.4 mmol of Co

(NO3)2·6H2O (Sigma-Aldrich) were mixed in a beaker

contain-ing 10 ml of an aqueous solution of 1 M citric acid (1 : 1 molar ratio of total metals to citric acid). The solution was kept under agitation to dissolve the nitrates at room temperature and NH3(30%) was added dropwise to adjust the pH to 7. The

solution was heated on a hot plate to 150 °C to form a gel, aided by the chelating action of the citric acid. After approxi-mately 1 h, the entire solution was completely converted to a dry gel, after which the temperature was rapidly increased to 300 °C inducing a flameless self-combustion, which was com-pleted in a few minutes. The final sample was then obtained in the form of a dry powder. Note that no further annealing was required. In order to prepare nickel ferrite nanoparticles (NFO), the same procedure was employed, substituting 3.4 mmol of Ni(NO3)2·6H2O in place of the Co(NO3)2·6H2O

precursor.

2.1.2 BiFeO3 and LaFeO3 matrix. In a typical synthesis

process of BFO, 0.00143 mol of Bi(NO3)3·5H2O, 0.00143 mol of

Fe(NO3)3·9H2O, and 0.00286 mol of glycine (Sigma-Aldrich)

were dissolved in a mixture of 14.3 mL deionized water and 1 mL HNO3 (65%), under magnetic stirring at ∼80 °C. The

clear solution was stirred at 80 °C for 20 min. Next, the temp-erature was increased and maintained at∼150 °C until the for-mation of a gel. The temperature was then increased to ∼250 °C when a self-combustion reaction with a flame occurred, yielding a fluﬀy powder. The powder was crushed in a mortar, transferred to a furnace, and annealed in air at 350 °C for 1 h, followed by further annealing at 500 °C for an additional 1 h. The sample annealed at 500 °C contained a small amount of Bi2O3impurity that can be removed either by

washing with acetic acid or by increasing the annealing temp-erature to 600 °C.

In a typical synthesis process of LFO, stoichiometric amounts of La(NO)3·6H2O and Fe(NO3)3·9H2O were dissolved

in deionized water under magnetic stirring at room tempera-ture (ratio of total weight of precursors (in g) : volume of water (in mL) = 1 : 1). To this solution, ethylene glycol (volume ratio of water : ethylene glycol = 1 : 1.5) was added. The clear solu-tion was then heated to 80 °C and stirred for 20 min. Next, the temperature was increased and maintained at ∼150 °C until the formation of a gel. The temperature was then increased to ∼250 °C when a self-combustion reaction with a flame occurred, yielding a fluﬀy powder. The powder was crushed in a mortar, transferred to a furnace, and annealed in air at 350 °C for 1 h, followed by further annealing at 450 °C for an additional 1 h, and finally at 500 °C for 10 h.

To determine the minimum annealing temperature necess-ary to obtain a pure phase (that turned out to be 500 °C in our case), the powder obtained after self-combustion was annealed at progressively higher temperatures (in steps of 50 °C i.e., at 350 °C, 400 °C, 450 °C, 500 °C, and so on) and, after each annealing, X-ray diﬀraction was performed to check the phase purity.

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2.1.3 Nanocomposites. The nanocomposites were syn-thesized by first dispersing an appropriate amount of CFO (or NFO) nanoparticles (synthesized as per the process described in section 2.1.1) in water and sonicating it for ∼30 min. Stoichiometric amounts of the respective precursors (of BFO or LFO) were then added, and the exact same steps as described in section 2.1.2 were followed for the synthesis of CFO/BFO (or NFO/BFO) and CFO/LFO (or NFO/LFO) nano-composites. The amounts of CFO and NFO were chosen such that the weight % of CFO (or NFO) in the nanocomposite cor-responded to 2% i.e., CFO (or NFO) : BFO (or LFO) = 2 : 98, by weight. In Fig. 1, we show a flowchart describing the synthesis process of the nanocomposites. A more detailed schematic description of the entire synthesis process is shown in Fig. S1 (in the ESI†).

2.2 Characterization techniques

The samples were characterized by X-ray powder diﬀraction (XRPD) obtained using a D-5000 diﬀractometer with CuKα

radiation operating at 40 kV and 30 mA. The data were col-lected in the range of 2θ = 20–70°, with a step size of 0.02°. The XRPD patterns were also used to estimate the average crys-tallite size of the samples, using the Williamson–Hall plot.44

The composition of the synthesized powders was investi-gated by energy dispersive spectroscopy (EDS) using a Zeiss SUPRA40 scanning electron microscope (SEM) equipped with a Bruker QUANTAX microanalysis system. Before observations, samples were prepared by spreading some powder on an adhesive conductive carbon disc attached to an SEM alu-minium stub.

Inductively coupled plasma optical emission spectroscopy (ICP-OES) was carried out for elemental analysis with an iCAP 6300 DUO ICP-OES spectrometer (ThermoScientific). The samples were digested in aqua regia (HCl 37% v/v– HNO369%

v/v 3 : 1) for 8 h, then diluted using Milli-Q water, and analysed.

Diﬀerential thermal analysis (DTA)/thermogravimetric ana-lysis (TGA) were performed using a LabsysEvo 1600 DTA/TGA (Setaram). 10 mg of sample obtained after self-combustion was put in an alumina crucible and heated from 30 to 1000 °C, at 10 °C min−1under an O2 atmosphere (20 ml min−1). The

DTA and TGA curves were elaborated using the dedicated soft-ware Calisto (Setaram).

Transmission electron microscopy (TEM) analysis was per-formed using a Philips CM200 microscope operating at 200 kV and equipped with a LaB6 filament. For TEM observations, the samples, in the form of powder, were prepared using the fol-lowing procedure. A small quantity of powder was dispersed in isopropyl alcohol and subjected to ultrasonic agitation for approximately one minute. A drop of suspension was de-posited on a commercial TEM grid covered with a thin carbon film; finally, the grid was kept in air until complete evapor-ation of the isopropyl alcohol.

Temperature and magnetic field-dependent magnetization of the samples was collected using a superconducting quantum interference device (SQUID) magnetometer from

Quantum Design Inc. The temperature dependence of magne-tization was recorded under zero-field cooled (ZFC) and field cooled (FC) conditions under a magnetic field H = 0.05 T. Magnetic hysteresis loops were recorded at T = 5 K in the−5 T to +5 T field range.

3. Results and discussion

3.1 Structural and morphological characterization

The XRPD patterns of the synthesized samples are shown in Fig. 2 and S2 (in the ESI†). The reflections of the cubic CFO (Fig. 2a) and NFO (Fig. 2d) could be indexed to the cubic space group (s.g. Fd3ˉm). BFO (Fig. 2b) and LFO (Fig. 2e) crystallize in

Fig. 1 Flowchart showing the synthesis process of the nanocomposites.

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the rhombohedral (s.g. R3c) and orthorhombic (s.g. Pnma) structures, respectively. Fig. 2c, 2f, S2a (in the ESI), and S2b (in the ESI†) show the XRPD patterns of the nanocomposites with 2% CFO (or NFO). The CFO and NFO phases either remain undetected in the XRPD patterns (Fig. 2c and S2a†) or only the most intense reflection (311) of the two phases can be observed (Fig. 2f and S2b†). This can be attributed to the extre-mely low volume % of CFO and NFO in the nanocomposites (∼2% in weight). All the observed reflections in the XRPD pat-terns could be indexed to the expected phases, and no impur-ity or secondary phase was observed. The average crystallite size (DXRPD) of the samples, as calculated from the XRPD data

using the Williamson–Hall plot,44 varied between 20–25 nm, and have been indicated in the corresponding panels in Fig. 2 and S2 (in the ESI†). The average crystallite size of the samples is also tabulated in Table 1. The estimation of strain from the Williamson–Hall plot shows that the strain does not change appreciably in the nanocomposites as compared to that of the constituent components. The reason for this could be the extremely small amount of the embedded component (only ∼2%) in the nanocomposites.

In order to investigate the chemical composition of the syn-thesized nanoparticles, EDS measurements were performed. The stoichiometry of the as-prepared NFO, CFO, and LFO samples was confirmed by EDS analysis. It is worth mention-ing here that the synthesis of BFO was more complicated than the synthesis of LFO because of the presence of an impurity phase of Bi2O3 in the as-prepared BFO sample annealed at

500 °C. This impurity phase could be eliminated by a proper choice of the chelating agent, and by performing the final annealing at 600 °C. However, annealing at higher temperature led to an increase in the average crystallite size from∼25 nm

in the sample annealed at 500 °C to∼43 nm in the sample annealed at 600 °C. To avoid such an increase in the crystallite size, we adopted an alternative route to eliminate the Bi2O3

impurity phase i.e., by washing the as-prepared sample with acetic acid. However, it is important to remember that we had started the synthesis with stoichiometric proportions of the Bi and Fe precursors, and the presence of a Bi2O3impurity phase

might mean that the Bi : Fe ratio in the BFO phase is diﬀerent from the desired ratio of 1 : 1. Concerning the BFO synthesis, EDS measurements were used to optimize the procedure. It was observed that when ethylene glycol was used as the chelat-ing agent durchelat-ing the synthesis process, the final atomic con-centrations of the powder, after washing with acetic acid, exhibited an increase in the Fe content with respect to the Bi content by approximately (5 ± 2)%. This observation can be explained by the formation of the Bi2O3 impurity phase, and

its subsequent removal by washing with acetic acid (see Fig. S3 in the ESI† for the corresponding XRPD results), leading to the

Fig. 2 XRPD patterns of (a) CFO, (b) BFO, (c) CFO/BFO nanocomposites, (d) NFO, (e) LFO, and (f ) NFO/LFO nanocomposites.

Table 1 Average crystallite size estimated from XRPD〈DXRPD〉,

magneti-zation recorded at 5 T and 5 K (M5 T), and coerciveﬁeld at 5 K (HC5 K)

(errors on the last digit are reported in parenthesis)

Sample 〈DXRPD〉 (nm) M5 T(Am2kg−1) HC5 K(T) BFO 25 (2) 1.2 (1) 0.20 (1) LFO 20 (1) 2.4 (2) 0.08 (1) CFO 21 (2) 64.0 (1) 1.10 (1) NFO 20 (1) 40.9 (2) 0.02 (1) CFO/BFO 25 (2) 2.7 (2) 0.60 (1) NFO/BFO 25 (2) 2.6 (1) 0.03 (1) CFO/LFO 20 (2) 7.0 (1) 0.25 (1) NFO/LFO 20 (2) 5.8 (2) 0.06 (1)

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formation of a non-stoichiometric BFO phase. The amount of Bi2O3in the as-prepared sample could be reduced significantly

by using glycine as the chelating agent instead of ethylene glycol (see Fig. S4 in the ESI† for the XRPD patterns). EDS measurements performed on the latter samples (after washing with acetic acid) have confirmed the formation of the stoichio-metric BFO phase, even for the sample annealed at 500 °C, thus, indicating that the amount of Bi2O3 formed in this

sample is small, and does not aﬀect the stoichiometry of the BFO phase appreciably. In order to confirm the weight percent of the spinel ferrite phase in LFO and BFO matrices, ICP ana-lysis was performed on two representative samples, i.e., CFO/ BFO and CFO/LFO nanocomposites. Focusing on the Co/Bi and Co/La ratios, weight fractions of 3.1% and 3.9% have been determined for the CFO/BFO and CFO/LFO nanocomposites, respectively. Taking into account the experimental errors, this value can be considered to be in good agreement with the nominal ones. In addition, DTA/TGA (not reported here) has been performed on the LFO/CFO nanocomposite under an O2

atmosphere. A first weight loss was observed between∼35 and ∼210 °C, equal to ∼9%, due to some residual solvent and adsorbed moisture. Next, the TG curve showed sequential steps for a total weight loss of ∼25% up to 500 °C, and an additional small weight loss (<5%) was observed up to 700 °C. The simultaneously recorded DTA curve revealed a broad exothermic peak in the temperature range of 200–500 °C, suggesting that the weight loss can be ascribed mainly to the combustion of carbonaceous residuals trapped in the compo-sites in some way.45

It is worth noting the importance of a proper choice of the chelating agent during synthesis. Diﬀerent surfactants and chelating agents have been used in sol–gel synthesis, for example, oleic acid,46,47 polyvinylpyrrolidone,48 hexamethyl-enetetramine,49 ethanolamine,50 ethyl oleate,51 and sodium dodecyl benzene sulfonate.52In our work, we used diﬀerent chelating agents to optimize the synthesis. Our experiments revealed that, in the case of CFO and NFO, the use of citric acid enabled phase formation at a temperature as low as 300 °C without the need for any further annealing. In the case of the matrix, ethylene glycol worked well for the synthesis of LFO. However, when we tried to synthesize BFO using ethylene glycol as the chelating agent, a large fraction of Bi2O3 was

obtained as an impurity phase. The fraction of Bi2O3could be

significantly reduced when ethylene glycol was replaced with glycine as the chelating agent.

The morphology of the samples was investigated by trans-mission electron microscopy; however, due to the low fraction of CFO (and NFO) in the nanocomposites, obtaining infor-mation about the morphology and homogeneity of the samples is not trivial. Hence, to facilitate imaging using TEM, we synthesized an additional nanocomposite sample (CFO/ LFO) with 10% CFO (see Fig. S2c in the ESI† for the XRPD pattern). Fig. 3a shows a typical bright field TEM image of the sample. The powder is composed of aggregates with size ranging from 200 nm to 3μm. The agglomerates are made of highly interconnected nanocrystals that give rise to porous

structures (illustrative TEM images are shown in Fig. S5 in the ESI†). Selected area electron diﬀraction (SAED) measurements performed on numerous agglomerates have revealed the crys-talline nature of the sample. Fig. 3b shows a dark field image of the sample shown in Fig. 3a. The image was obtained by selecting diﬀraction spots corresponding to the {121}LFO

family planes, and in this way, the LFO nanocrystals respon-sible for the corresponding diﬀraction spots are virespon-sible. The SAED pattern shown in Fig. 3c was obtained on a portion of the sample shown in Fig. 3a. The major part of the visible diﬀraction spots is due to the LFO phase and only two spots can be attributed to the CFO phase (white arrows: {111}CFO

family planes). All the SAED measurements show very few diﬀraction spots generated by the CFO phase. This result agrees with the small proportion of the CFO phase with respect to the LFO phase (10 : 90 by weight), indicating a very good dispersion of the CFO nanocrystals. Indeed, the presence of clusters of nanocrystals would have generated larger di ﬀrac-tion signals.

One of the CFO nanocrystals is shown in the high-resolu-tion TEM image in Fig. 3d (white outlined area). The distance d among the visible planes, calculated by the fast Fourier trans-form (FFT) of the image, is 0.484 nm corresponding to the {111}CFOinterplanar distance.

In order to confirm further the existence and good dis-persion of the embedded magnetic phase, a nanocomposite sample (CFO/LFO) with 50% CFO was synthesized. The larger amount of the magnetic phase enabled the TEM analysis to clearly confirm the uniform distribution of CFO and LFO nanocrystals (Fig. S6 in the ESI†). We have also shown typical high resolution TEM images showing the dimensions and structure of few LFO and CFO nanocrystals with the corres-ponding fast Fourier transforms shown in Fig. S7 and S8 in the ESI.†

3.2 Magnetization measurements

Magnetization versus temperature curves of all the synthesized samples were measured in the temperature range of 5–400 K (Fig. S9 and S10 in the ESI†). However, since the ordering temperatures of all the samples lie above 400 K, we were unable to track the variation in the ordering temperatures. Nevertheless, low temperature magnetic field-dependent mag-netization measurements were extremely insightful, as elabo-rated below.

Field dependence of magnetization of the synthesized samples recorded at T = 5 K are shown in Fig. 4 and S11 (in the ESI†). We first focus on the curves shown in the left panel of Fig. 4 i.e., the isothermal magnetization curves of CFO (Fig. 4a), BFO (Fig. 4b), and CFO/BFO composites (Fig. 4c). At high magnetic fields (H = 5 T), the magnetization of the CFO sample reaches∼64 Am2kg−1. BFO, on the other hand, is anti-ferromagnetic, and hence, the magnetization shows no sign of saturation even at high magnetic fields, reaching a value of ∼1.2 Am2_kg−1_{at H = 5 T. The non-zero values of the magnetic}

remanence and coercivity can be attributed to its weakly ferro-magnetic nature arising from finite size eﬀects.53_{Nevertheless,}

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the high field magnetization and coercivity of BFO are con-siderably smaller than the corresponding values of CFO. Indeed, our experiments revealed that any weight fraction of CFO >2% resulted in the magnetic behavior of the nano-composite sample being completely dominated by the CFO phase to the extent that the BFO phase was entirely oversha-dowed (see Fig. S12 in the ESI† for a comparison of the iso-thermal magnetization curves of pure CFO and CFO/LFO nano-composites with 10% CFO and 50% CFO).

It is important to note here that despite the presence of two phases in the nanocomposite samples, their magnetic behav-ior is reminiscent of single phase-like behavbehav-ior. This is unlike samples prepared via simple physical mixing of the two phases, or even some core–shell nanoparticles where anoma-lous hysteresis behavior has been reported and attributed to the lack of coupling between the two phases.33,54 In certain cases, the lack of magnetic coupling in physically mixed samples manifests itself in the form of an anomalous hystere-tic behavior where the loop shape of the physically mixed sample is close to a simple superposition of the hysteresis loops of the pure individual phases.33 However, in our case,

the saturation magnetization values of the two individual com-ponents (BFO (LFO) and CFO (NFO)) are diﬀerent by an order of magnitude (Fig. 4). This makes it impossible to see the anomalous hysteretic behavior in the loop shape of the phys-ically mixed samples of BFO (LFO) and CFO (NFO) (not shown here).

A more eﬀective comparison of the magnetic characteristics of the composite vis-à-vis that of pure BFO and CFO is possible when we plot the isothermal magnetization curves after nor-malizing the magnetization axis with respect to the magnetiza-tion value at H = 5 T, as shown in Fig. 5a. As is evident from the figure, the coercivity of the composite and its normalized remanence (i.e. Mr/M5 T) value are intermediates between the

corresponding values of BFO and CFO. In other words, we have increased the values of the coercivity and normalized remanence of the composite with respect to that of the BFO matrix.

Next, we investigate what happens when we replace CFO with a ferrite that has a much lower coercivity (i.e., magnetic anisotropy), in this case, NFO. While CFO has a coercivity (HC–CFO∼1.1 T) that is one order larger than that of the BFO

Fig. 3 CFO/LFO (10 : 90 by weight) nanocomposite: (a) TEM bright_{field image, (b) TEM dark field image showing LFO nanocrystals, (c)} corres-ponding SAED pattern where the {111}CFOdiffraction spots are shown by arrows, and (d) high-resolution TEM image of a CFO nanocrystal.

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Fig. 4 Isothermal magnetization curves of (a) CFO, (b) BFO, (c) CFO/BFO composite, (d) NFO, (e) LFO, and (f ) NFO/LFO composite recorded at T = 5 K.

Fig. 5 Normalized isothermal magnetization curves of (a) CFO (red triangles), BFO (black circles), and CFO/BFO composite (orange inverted tri-angles), and (b) NFO (green diamonds), BFO (black circles), and NFO/BFO composite (orange inverted triangles) recorded at_{T = 5 K (expanded} region around the origin). The inset in (b) shows the extendedﬁeld range from −5 T to +5 T.

Fig. 6 Normalized isothermal magnetization curves of (a) CFO (red triangles), LFO (blue circles), and CFO/LFO composite (orange inverted tri-angles), and (b) NFO (green diamonds), LFO (blue circles), and NFO/LFO composite (orange inverted triangles) recorded at_{T = 5 K (expanded region} around the origin). The inset in (b) shows the extended_{ﬁeld range from −5 T to +5 T.}

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matrix (HC–BFO∼0.2 T), the coercivity of NFO (HC–NFO∼0.02 T)

is one order smaller than that of the BFO matrix. As is clear from Fig. 5b, the NFO/BFO composite also enables us to tune the coercivity of the composite with respect to that of the BFO matrix.

In order to examine whether the aforementioned smooth tuning of the coercivity is specific to the case of BFO, or is a more general trait, we have performed similar experiments with the CFO/LFO and NFO/LFO nanocomposites. The results are illustrated in Fig. 6a and b, where we show the normalized isothermal magnetization curves of LFO, CFO, NFO, and the two composites (CFO/LFO and NFO/LFO). A similar tuning of the coercivity with respect to that of the LFO matrix (increase or decrease depending on whether the composite is syn-thesized using CFO or NFO nanoparticles) is observed, con-firming that our results represent a general feature of composites prepared using the synthesis technique described before.

The magnetic coupling in the nanocomposites can be qualitatively evaluated by remanent magnetization using direct current demagnetization (DCD) experiments.33,55 The DCD curve is obtained by first saturating the sample in a negative field, and then measuring the magnetization at zero field after applying fields of increasing amplitude. The diﬀerentiated curve of MDCD with respect to H represents the irreversible

component of susceptibility (χirr). This quantity can be

con-sidered to be a measure of the energy barrier distribution which, in a nanoparticle system, is associated with a particle’s switch-ing field distribution (SFD), defined as the field necessary to overcome the energy barrier during an irreversible reversal process.56 In Fig. 7, we show the measured MDCD curves of

LFO, CFO, and CFO/LFO nanocomposites (in the inset), and the corresponding diﬀerentiated curves (in the main panel).

As is clear from the figure, for LFO and CFO, we see strong contributions centered around a single peak (at ∼0.03 T for LFO and at∼1.0 T for CFO, indicated by blue and red arrows, respectively). In contrast, in the nanocomposite, the SFD is

Fig. 7 Inset: Normalized_MDCDversus reverse magnetic ﬁeld of CFO (red triangles), LFO (blue circles), and CFO/LFO composite (orange inverted

tri-angles). Main panel: Corresponding switching_{ﬁeld distributions as obtained from the ﬁrst order derivatives of the M}DCDcurves.

Fig. 8 Schematic representation of the tuning of coercivity by synthe-sizing nanocomposites. The coercivity values correspond to actual experimental values measured atT = 5 K.

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centered around two peaks (orange arrows), indicating the individual contributions of the two phases. However, the two peaks do not occur at the same fields as in LFO and CFO. Instead, they are shifted closer with respect to the original pure phases (indicated by the two orange arrows at∼0.1 T and 0.9 T in Fig. 7). This shift indicates the presence of magnetic coupling between the two phases, since the reversal of the two magnetic phases is not independent.55

A schematic representation of the coercivity-tuning via synthesizing nanocomposites is shown in Fig. 8. Note that the coercivity values in the figure correspond to actual experi-mental values measured at T = 5 K. The magnetic components (CFO and NFO) were chosen such that one had a coercivity much higher than that of the matrix (BFO or LFO) and the other had a coercivity much lower than that of the matrix, so that we could effectively demonstrate the tuning of the coerciv-ity. The tuning of the coercivity due to formation of compo-sites is, in principle, independent of the measuring tempera-ture, and should occur at T = 300 K also. However, the phenomenon is not clearly visible from the room temperature M–H loops due to the comparable values of coercivity of the individual phases at room temperature. Hence, we have demonstrated this effect using low-temperature M–H loops where the effect is clearly visible.

4. Conclusions

In summary, we have synthesized magnetic nanocomposites using CFO or NFO as one component and BFO or LFO as the second component (matrix) via a novel chemical synthesis route. Our synthesis route yields superior quality samples with single-phase-like magnetic behavior as compared to samples prepared via simple physical mixing of the two phases that have been reported to lack enough coupling between the two phases. Furthermore, we demonstrate a striking tuning of the magnetic coercivity with respect to that of the matrix depend-ing on the choice of the embedded component in the nano-composites. We believe that these results will be of huge value in the quest for nanocomposites with functional properties.

Con

ﬂicts of interest

There are no conflicts of interest to declare.

Acknowledgements

This work has been performed in the framework of the grant CTS 16:306 from the Carl Tryggers Stiftelse för Vetenskaplig Forskning. The Swedish Research Council (VR) is thanked for financial support. TS acknowledges support from the VR start-ing grant 2017-05030; DP and RM thank the Wenner-Gren Foundations for financial support.

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